A strain gauge is a device for measuring dimensional change primarily on the surface of a specimen as the latter is subjected to mechanical, thermal, or a combination of both stresses. One type of strain gauge is attached to the specimen surface and mechanically amplifies the surface distortion so that the change can be measured on a simple dial indicator. Other types of strain gauges measure the displacement of light rays through an optical system that is actuated by the surface strain or convert this strain into an electrical signal. The mechanical, electromechanical, and optical strain gauge devices are considered extensometers, and their use is generally limited to calibration or to the testing of materials' properties.
The electrical type of strain gauge is in wide use today and has found applications far beyond those of a conventional extensometer. Strain/stress sensors that give an electrical output that relates to the strain/stress are increasingly important due to the advent of smart structures, such as those discussed in U.S. Pat. No. 5,440,300 to Spillman, Jr., which require strain/stress sensing either to monitor structural health or to control the structure.
Electrical-type strain gauges can be based upon the measurement of a change in capacitance, inductance, resistance, or dipole moment that is proportional to strain. The principle of a resistance-type strain gauge can be illustrated with a rod-shaped conductor whose resistivity, .rho., remains reasonably constant over the range of strains encountered. As the rod is elongated in response to tensile stress, the length ("L") of the rod increases and its cross-sectional area ("A") decreases. Since the resistance ("R") of a conducting varies directly with the conducting rod's length and inversely with its cross-sectional area according to the formula R=.rho.L/A, elongation causes an increase in the rod's resistance. The resistance change (".DELTA.R/R.sub.o "), is related to the length change (i.e., strain, ".DELTA.L/L"), by the formula: .DELTA.R/R.sub.o =.epsilon.(.DELTA.L/L), .epsilon. is the strain sensitivity or gauge factor. Materials, such as the metal rod used above to illustrate the concept, which exhibit changes in their electrical resistance due to strain/stress are said to be piezoresistive.
Semiconductors, the energy band gap of which changes with strain/stress (Gridchin et al., Sensors Actuators A, 49:67-72 (1995)) represent another type of piezoresistive material. The most common type of piezoresistive material is a composite material with an electrically non-conducting matrix, usually a polymer, and a conducting filler. Conventional fillers include: carbon fibers having a 10 .mu.m diameter (Muto et al., J. Ceramic Soc. Jpn., 100:582-585 (1992) and Pramanik et al., J. Mat. Sci., 25:3848-3853 (1990) ("Pramanik")); carbon black (Pramanik, Kost et al., J. Appl. Polymer Science, 29:3937-3946 (1984), and Radhakrishnan et al., Mater. Lett., 18:358-362 (1994) ("Radhakrishnan")); and metal particles (Radhakrishnan). Upon tension, the distance between adjacent filler units increases, so the resistivity increases; upon compression, the distance between adjacent filler units decreases, so the resistivity decreases. Thus, in these materials, the change in electrical resistance is due not only to a change in the material's dimensions but also to a change in its electrical resistivity.
One shortcoming of conventional piezoresistive strain sensors, such as those described above, is the non-linearity between the resistivity change and the strain which most of these piezoresistive strain sensors exhibit. Little attention has been paid to the alleviation of this problem via the design of the composite. Conventional piezoresistive strain sensors also suffer from low strain sensitivities, making them unsuitable for applications where strains and stresses are small. Moreover, conventional piezoresistive strain sensors, especially those which employ a filler in a polymeric matrix, do not have the chemical and thermal resistance needed in many applications.
Therefore, there is a continuing need for new methods for measuring strain/stress, strain/stress sensors, and compositions useful therefor. The present invention is directed to these methods, sensors, and compositions.